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Chandra opens new line of investigation on dark energyNASA NEWS RELEASEPosted: May 18, 2004

Astronomers have detected and probed dark energy by applying a powerful, new method that uses images of galaxy clusters made by NASA's Chandra X-ray Observatory. The results trace the transition of the expansion of the universe from a decelerating to an accelerating phase several billion years ago.

Chandra images of multimillion degree Celsius gas in galaxy clusters have provided astronomers with a powerful new method to probe the mass and energy content of the universe. A recent study of 26 clusters of galaxies confirms that the expansion of the universe stopped slowing down about 6 billion years ago, and began to accelerate. Credit: NASA/CXC/IoA/S.Allen et al.Download larger image version here

"Dark energy is perhaps the biggest mystery in physics," said
study leader Steve Allen of the Institute of Astronomy (IoA)
University of Cambridge, England. "As such, it is extremely
important to make an independent test of its existence and
properties," he said.

Allen and his colleagues used Chandra to study 26 clusters of
galaxies at distances between one and eight billion light
years. These data span the time when the universe slowed from
its original expansion, before speeding up again, because of
the repulsive effect of dark energy.

"We're directly seeing the expansion of the universe is
accelerating by measuring the distances to these galaxy
clusters," said IoA scientist and study co-author Andy
Fabian. "The new Chandra results suggest the dark energy
density does not change quickly with time and may even be
constant, consistent with the "cosmological constant" concept
first introduced by Albert Einstein," he said.

If the dark energy is unchanging, the universe is expected to
continue expanding forever, and more dramatic fates for the
universe would be ruled out. These include the "Big Rip,"
where dark energy increases until galaxies, stars, planets
and, finally, even atoms are torn apart, and the "Big
Crunch," where the universe eventually collapses on itself.

Chandra's probe of dark energy uses X-ray observations to
detect and study the hot gas in galaxy clusters. From these
data, the ratio of the mass of the hot gas to the mass of the
dark matter in a cluster can be determined. Since galaxy
clusters are so large, the relative amounts of hot gas and
dark matter should be the same for every cluster. Using this
assumption, Allen and colleagues derive distances that show
the expansion of the universe was first decelerating, and it
began to accelerate about six billion years ago.

Chandra's observations agree with observations of distant
supernovae, including those from NASA's Hubble Space
Telescope (HST), that first showed dark energy's effect on
the acceleration of the universe. Chandra's results are
completely independent of the supernova technique.

"Our Chandra method has nothing to do with other techniques,
so they're definitely not comparing notes, so to speak," said
Robert Schmidt of the University of Potsdam, Germany, another
co-author of the study.

Better limits on the amount of dark energy, and how it varies
with time, are obtained by combining the X-ray results with
data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP).
It used observations of the cosmic microwave background
radiation to discover evidence for dark energy in the very
early universe. Using the combined data, Allen and his
colleagues found dark energy makes up about 75 per cent of
the universe, dark matter about 21 per cent, and ordinary
matter about 4 per cent.

More detailed studies with Chandra, HST, WMAP and future X-
ray missions like Constellation-X, should provide much more
precise constraints on dark energy.

"Until we better understand cosmic acceleration and the
nature of the dark energy, we cannot hope to understand the
destiny of the universe," said Michael Turner, assistant
director for mathematical and physics sciences, National
Science Foundation, Arlington, Va.

The research team also included Harald Ebeling of the
University of Hawaii and the late Leon van Speybroeck of the
Harvard-Smithsonian Center for Astrophysics. These results
appear in an upcoming issue of the Monthly Notices of the
Royal Astronomy Society.

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